Recent developments in preparation of activated carbons by microwave: Study of residual errors

2.1 Preparation of adsorbent and adsorbate

The leaves of A. nilotica were dried in an oven and treated with conc. H₂SO₄ for 12 h and washed thoroughly with distilled water till neutral pH is attained and soaked in 2% NaHCO₃ overnight in order to remove any excess of acid present. Then the material (CVM) was washed with distilled water and dried. The raw sample is placed in a microwave oven (Samsung; Triple Distribution System) at 800 W for 2 min. The carbonized sample (MVM) was collected. Both CVM and MVM were preserved in an air tight container for further studies.

The commercial grade crystal violet (color index No. 42555) with molecular formula C₂₅H₃₀ClN₃, molecular weight 407.99 and λ_max 584 nm are obtained from Thomas baker (chemicals) Ltd., Mumbai, India (Fig. 1). All the chemicals used throughout this study were analytical-grade reagents and the adsorption experiments were carried out at room temperature (27 ± 2 °C).

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Figure 1

Structure of crystal violet dye.

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2.2 Characterization of CVM and MVM

The surface morphology and fundamental physical properties of the sorbents were obtained by the scanning electron microscope (LEO 435 VP model) and Fourier Transform Infrared (FTIR) analysis. Determination of zero point charge (pH_zpc) was done to investigate the surface charge of both chemically and microwave activated adsorbents for different solution pH.

2.3 Batch adsorption studies

The adsorption experiments were carried out in a batch process to evaluate the effect of pH, contact time, adsorbent dose, adsorption kinetics, adsorption isotherm and desorption of CV onto CVM and MVM.

3.1 Characterization of CVM and MVM

A SEM study shows (Figs. 2a and 2b) that both CVM and MVM have considerable number of heterogeneous pores where there is a good possibility for dye to be trapped and adsorbed (Hameed and El-Khaiary, 2008). This figure reveals that the MVM shows much more rough irregular in shape and porous as compared to CVM.

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Figure 2a

SEM image of CVM.

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Figure 2b

SEM image of MVM.

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The FTIR spectra of CVM and MVM are presented in Figs. 3a and 3b. The band at 3412 cm⁻¹ represents the presence of –OH and –NH groups. The band observed at 2926 and 2856 cm⁻¹ is associated to asymmetric and symmetric stretching vibrations of –CH group of both carbons (Ahmad and Kumar, 2010). Atmospheric CO₂ bands are present at 2360 cm⁻¹ (antisymmetric C⚌O stretch) was observed for MVM only. The bands at 1637 and 1452 cm⁻¹ indicate the presence of –COO, –C⚌O and –NH groups for CVM and MVM. The peak at 1564 cm⁻¹ is attributed to the formation of oxygen functional groups such as a highly conjugated C–O stretching in carboxylic groups (Freundlich, 1960) and due to the presence of quinone structure. The bands at 1390–1408 cm⁻¹ are assigned to C–H bending in alkanes or alkyl groups were observed in MVM. The broad band between 1325 and 1109 cm⁻¹ has been assigned to C–O stretching in alcohols and phenols (Freundlich, 1960; Shelden et al., 1993). Another absorption band appearing around 1261 and 663 cm⁻¹ can be attributed to the Si–C stretch and C–O–H twist. The band at 1261 cm⁻¹ implied the C–N stretching of amide similarly present in MVM. The characteristics of CVM and MVM are presented in Table 1.

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Figure 3a

FTIR spectra of CVM.

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Figure 3b

FTIR spectra of MVM.

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3.2 Effect of pH on the adsorption of CV onto CVM and MVM

The adsorption capacity increased with increase in solution pH and the maximum adsorption capacity for CV was observed at 5 pH onto CVM and 6 pH onto MVM depicted in Fig. 4a. The effect of solution pH is very important when the adsorbing molecules are capable of ionizing in response to the current pH. When solution pH increases, high OH⁻ ions accumulate on the adsorbent surface (Ahmad and Kumar, 2010). Therefore, electrostatic interaction between negatively charge adsorbent surface and cationic dye molecules increases the adsorption (Santhi et al., 2010). Furthermore, the solution pH is above the zero point charge (pH_zpc = 3.8 for CVM and 4.4 for MVM) and hence the negative charge density of the surface of the adsorbents increased which favors the adsorption of cationic dye (Janos et al., 2003). The optimum pH value of CV onto CVM and MVM was 5 and 6 pH, respectively.

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Figure 4a

Effect of pH on the adsorption of CV onto CVM and MVM.

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3.3 Effect of contact time and initial dye concentration

The uptake of CV onto CVM and MVM as a function of contact time is shown in Figs. 4b and 4c. It can be seen that the amount of CV adsorbed per unit mass of adsorbent increased with increasing dye concentration, although percentage removal decreased with increase in initial dye concentration. The uptake of CV was increased from 6.83 to 19.97 mg g⁻¹ for CVM and 6.89 to 22.56 mg g⁻¹ for MVM with the increase in CV concentration from 50 to 200 mg L⁻¹. This may be attributed to an increase in the driving force of the concentration gradient with the increase in the initial dye concentration (Hameed et al., 2008).

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Figure 4b

Effect of contact time and initial concentration on the adsorption of CV onto CVM.

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Figure 4c

Effect of contact time and initial concentration on the adsorption of CV onto MVM.

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The uptake of CV onto CVM and MVM was increased with the increase in contact time. The adsorption equilibrium was achieved 110 min for CVM and 90 min for MVM. So compared to MVM, CVM is more time consuming and also the percentage of dye removal is less. Initial adsorption was rapid due to the adsorption of dye onto exterior surface, after that dye molecules enter into pores (interior surface), relatively slow process (Ahmad and Kumar, 2010). Data on the adsorption kinetics of dyes by various adsorbents have shown a wide range of adsorption rates. For example, the effect of contact time for the adsorption of orange-G and crystal violet dye by bagasse fly ash was studied for a period of 24 h for initial dye concentrations of 10 mg L⁻¹ at 30 °C (Mall et al., 2006). The authors Mall et al. (2006) reported that after 4 h of contact, a steady-state approximation was assumed and a quasi-equilibrium situation was accepted.

3.4 Effect of adsorbent dose on dyes adsorption

The adsorption of CV onto CVM and MVM was studied by changing the quantity of adsorbent (0.2, 0.6 and 1 g) in the test solutions while keeping the initial dyes concentration 100 mg L⁻¹ (Fig. 4d). It was observed that the percentage removal increases with increase in adsorbent dose. The increase in % color removal was due to the increase of the available sorption surface and availability of adsorption sites (Hameed, 2009). A similar observation was previously reported from the removal of malachite green dye from aqueous solution by bagasse fly ash and activated carbon (Mall et al., 2005).

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Figure 4d

Effect of adsorbent dose on the adsorption of CV onto CVM and MVM.

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3.5 Adsorption isotherm

To optimize the design of an adsorption system for the adsorption of adsorbates, it is important to establish the most appropriate correlation for the equilibrium curves (Altinisik et al., 2010). The equation parameters of these equilibrium models often provide some insight into the sorption mechanism, the surface properties and the affinity of the adsorbent (Bulut et al., 2008). Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) were used to describe the equilibrium characteristics of adsorption.

3.5.1

3.5.1 The Langmuir isotherm

The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface (Langmuir, 1918). A basic assumption is that sorption takes place at specific homogeneous sites within the adsorbent. Once a dye molecule occupies a site, no further adsorption can take place at that site. The Langmuir adsorption isotherm has been successfully used to explain the adsorption of basic dyes from aqueous solutions (Tan et al., 2007; Hameed et al., 2007). It is commonly expressed as followed:

(10)

$$q_{\text{e}}=(Q_{\text{m}}{K}_{\text{a}}{C}_{\text{e}})/(1+K_{\text{a}}{C}_{\text{e}})$$ The Langmuir isotherm equation (10) can be linearized into the following form (Kinniburgh, 1986):

(11)

$$C_{\text{e}}/q_{\text{e}}=1/K_{\text{a}}{Q}_{\text{m}}+(1/Q_{\text{m}}\times C_{\text{e}})$$ where q_e and C_e are defined before in Eq. (1), Q_m is a constant and reflects a complete monolayer (mg g⁻¹); K_a is adsorption equilibrium constant (L mg⁻¹) that is related to the apparent energy of sorption. A plot of C_e/q_e versus C_e should indicate a straight line of slope 1/Q_m and an intercept of 1/(K_aQ_m).

The results obtained from the Langmuir are shown in Table 2 and the theoretical Langmuir isotherm is plotted in Figs. 5a and 5b together with the experimental data points. The related error function values (Table 3) are near the experimental adsorbed amounts and correspond closely to the adsorption isotherm plateau, which is acceptable. The correlation coefficient showed strong positive evidence on the adsorption of CV onto CVM and MVM follows the Langmuir isotherm. The applicability of the linear form of Langmuir model to CVM and MVM was proved by the high correlation coefficient R² = 0.9793 and 0.9998 for CV adsorption. This suggests that the Langmuir isotherm provides a good model of the sorption system. The maximum monolayer capacity Q_m obtained from Langmuir is 30.4878 mg g⁻¹ for CV onto CVM and 38.0228 mg g⁻¹ for CV onto MVM, respectively.

Table 2 Isotherm constants for adsorption of CV onto 0.2 g of CVM and MVM.

Isotherm model	CVM	MVM
Langmuir
Qm (mg g−1)	30.4878	38.0228
b (L mg−1)	0.0193	0.0158
R2	0.9793	0.9998
Freundlich
1/n	0.6313	0.9371
KF (mg g−1)	1.1920	1.0819
R2	0.9456	0.9717
Dubinin–Radushkevich
Qm (mg g−1)	16.8928	18.0307
K (×10−5 mol2 kJ−2)	2.0000	2.0000
E (kJ mol−1)	0.15811	0.15811
R2	0.8770	0.8318

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Figure 5a

Langmuir adsorption isotherm for CV onto CVM.

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Figure 5b

Langmuir adsorption isotherm for CV onto MVM.

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Table 3 Values of error function of the isotherm models of CV onto CVM and MVM.

Adsorbent	Error function model	Isotherm model
		Langmuir	Freundlich	D–R
CVM	RMSE	6.7015	22.2836	27.64
	χ2	0.16421	25.2747	52.3829
	ERRSQ	2.594	102.612	134.8966
	HYBRD	0.18285	7.23150	9.5066
	MPSD	0.0128	0.5096	0.66996
	ARE	0.1135	0.71388	0.8185
	EABS	1.6108	10.1298	11.6145
	APE	1.4189	8.9235	10.231
MVM	RMSE	7.3788	20.9144	31.5652
	χ2	0.1697	13.6556	46.78094
	ERRSQ	2.8905	84.1035	148.0079
	HYBRD	0.18855	5.48631	9.6549
	MPSD	0.01230	0.3588	0.62955
	ARE	0.11090	0.59823	0.79344
	EABS	1.70016	9.1708	12.16585
	APE	1.3863	7.47796	9.92016

3.5.2

3.5.2 The Freundlich isotherm

The Freundlich isotherm (Freundlich, 1960) can be applied to non-ideal adsorption on heterogeneous surfaces as well as multilayer sorption. The Freundlich isotherm can be derived assuming a logarithmic decrease in enthalpy of adsorption with the increase in the fraction of occupied sites and is commonly given by the following non-linear equation:

(12)

$$q_{\text{e}}=K_{\text{F}}{C}_{\text{e}}^{1/n}$$ Eq. (12) can be linearized in the logarithmic form (Eq. (13)) and the Freundlich constants can be determined:

(13)

$$\log q_{\text{e}}=\log K_{\text{F}}+\frac{1}{n}\log C_{\text{e}}$$

A plot of log q_e versus log C_e (Figs. 5c and 5d) enables to determine the constant K_F and 1/n. K_F is roughly an indicator of the adsorption capacity, related to the bond energy and 1/n is the adsorption intensity of dye onto the adsorbent or surface heterogeneity. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption. Value of 1/n < I represent favorable adsorption condition (Ho and McKay, 1978). The data obtained from linear Freundlich isotherm for the adsorption of the CV onto CVM and MVM is presented in Table 2. The comparability of the Freundlich model to the Langmuir model was proved by the correlation coefficient R² = 0.9456 and 0.9717 for adsorption of CV onto CVM and MVM. The 1/n value is lower than unity, indicating the favorable adsorption process as it is equal to 0.6313 for CV onto CVM and 0.9371 for CV onto MVM. It is lower than the experimental amounts corresponding to the adsorption isotherm plateau, which is unacceptable.

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Figure 5c

Freundlich adsorption isotherm for CV onto CVM.

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Figure 5d

Freundlich adsorption isotherm for CV onto MVM.

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3.5.3

3.5.3 The Dubinin–Radushkevich (D–R) isotherm

The D–R isotherm was also applied to estimate the porosity apparent free energy and the characteristics of adsorption (Dubinin, 1960, 1965; Radushkevich, 1949). It can be used to describe adsorption on both homogenous and heterogeneous surfaces (Shahwan and Erten, 2004). The D–R equation can be defined by the following equation:

(14)

$$\ln q_{\text{e}}=\ln Q_{\text{m}}-K{\epsilon }^2$$ where K is a constant related to the adsorption energy, Q_m the theoretical saturation capacity, ε the Polanyi potential, calculated from Eq. (15):

(15)

$$\epsilon =\mathit{RT}\ln \left(1+\frac{1}{C_{\text{e}}}\right)$$ where C_e is the equilibrium concentration of dye (mol L⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T is the temperature (K). By plotting ln q_e versus ε², it is possible to determine the value of K from the slope and the value of Q_m (mg g⁻¹) from the intercept (Figs. 5e and 5f). The mean free energy E (kJ mol⁻¹) of sorption can be estimated by using K values as expressed in the following equation (Hobson, 1969):

(16)

$$E=\frac{1}{\sqrt{2K}}$$ The parameters obtained using above equations was summarized in Table 2. The saturation adsorption capacity Q_m obtained using D–R isotherm model for CV onto CVM is 16.8928 mg g⁻¹ and MVM is 18.0307 mg g⁻¹. The E values calculated using Eq. (16) are 0.15811 kJ mol⁻¹ for both CV adsorption onto CVM and MVM. This was indicating that the physical adsorption plays the significant role in the uptake of CV onto CVM and MVM. The error functions indicate that the D–R isotherm is not fit in this adsorption. It can be seen that the Langmuir isotherm fits the data better than Freundlich and D–R isotherms. The fact that it may be due to the predominantly homogeneous distribution of active sites on the activated carbons surfaces; since the Langmuir equation assumes that the adsorbent surface is energetically homogeneous. This is also confirmed by the high value of R² (Table 2). This indicates that the adsorption of CV onto CVM and MVM takes place as monolayer adsorption.

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Figure 5e

D–R adsorption isotherm for CV onto CVM.

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Figure 5f

D–R adsorption isotherm for CV onto MVM.

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Over the past few decades, linear regression has been developed as a major option in designing the adsorption systems. However, recent investigations have indicated the growing discrepancy (between the predictions and experimental data) and disability of the model, propagating toward a different outcome. However the expanding of the non-linear isotherms represents a potentially viable and powerful tool, leading to the superior improvement in the area of adsorption science (Foo and Hameed, 2010).

The maximum sorption capacity (Q_m) of the adsorption of CV onto CVM and MVM was compared with those reported in literature (Table 4).

Table 4 Reported maximum adsorption capacities (Q_m in mg g⁻¹) in the literature for cationic dye obtained on low-cost adsorbents.

Adsorbent	Qm (mg g−1)	References
MVM	38.0228	This study
CVM	30.4878	This study
Luffa cylindrical	9.920	Altinisik et al. (2010)
Coffee ground activated carbon	23.00	Reffas et al. (2010)
Kolin	5.44	Gupta et al. (2008)
Wheat bran	22.73	Gupta et al. (2007)
Rice bran	14.63	Gupta et al. (2007)
Hen feathers	26.10	Mittal (2006)
Arundo donax root carbon	8.69	Zhang et al. (2008)
Bentonite	7.72	Tahir and Rauf (2006)

3.6 Adsorption kinetics

The kinetics of adsorbate uptake is important for choosing optimum operating conditions for design purposes. In order to investigate the mechanism of adsorption and potential rate controlling steps such as chemical reaction, diffusion control and mass transport process, kinetic models have been used to test experimental data from the adsorption of CV onto CVM and MVM. These kinetic models were analyzed using pseudo-first-order, pseudo-second-order and intraparticle diffusion.

3.6.1

3.6.1 Pseudo-first-order equation

The pseudo-first-order equation of Lagergren is generally expressed as follows:

(17)

$$\frac{{\mathit{dq}}_t}{d_t}=K_1(q_{\text{e}}-q_t)$$ where q_e and q_t (mg g⁻¹) are the adsorption capacity at equilibrium and at time t, respectively, k₁ (L min⁻¹) is the rate constant of pseudo-first-order adsorption. Integrating Eq. (17) for the boundary conditions t = 0–t and q_t = 0–q_t gives

(18)

$$\log \left(\frac{q_{\text{e}}}{q_{\text{e}}-q_t}\right)=\frac{K_1}{2.303}t$$ Eq. (18) can be rearranged to obtain the following linear form:

(19)

$$\log (q_{\text{e}}-q_t)=\log (q_{\text{e}})-\frac{K_1}{2.303}t$$ The values of q_e and K₁ for the pseudo-first-order kinetic model were determined from the intercepts and the slopes of the plots of log(q_e − q_t) versus time (Figs. 6a and 6b). The K₁ values, R² values and q_e values (experimental and calculated) are summarized in Table 5.

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Figure 6a

Pseudo-first-order kinetics at different initial concentrations for CV onto CVM.

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Figure 6b

Pseudo-first-order kinetics at different initial concentrations for CV onto MVM.

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Table 5 First- and second-order kinetic parameters for the adsorption of CV onto CVM and MVM at different initial concentrations.

Sorbent	C0 (mg L−1)	qe(exp) (mg g−1)	Pseudo-first-order			Pseudo-second-order
			K1 (min)	qe(cal) (mg g−1)	R2	K2 (g mg−1 min−1)	qe(cal) (mg g−1)	R2
CVM	50	7.5827	0.0196	2.0739	0.9636	0.0201	8.5763	0.9975
	100	14.4218	0.0189	1.6722	0.9706	0.02756	15.2207	0.9995
	150	18.3791	0.0168	1.3601	0.9710	0.0369	19.0476	0.9997
	200	20.6091	0.0147	1.6489	0.9851	0.0266	21.4132	0.9996
MVM	50	8.250	0.0269	3.7783	0.9577	0.0118	10.010	0.9977
	100	15.331	0.0285	4.3251	0.9123	0.0125	17.2117	0.9992
	150	19.413	0.0257	4.3471	0.8804	0.0113	21.3219	0.9991
	200	23.997	0.0244	2.9798	0.9729	0.0175	25.0629	0.9998

3.6.2

3.6.2 Pseudo-second-order equation

The pseudo-second-order equation is generally given as follows:

(20)

$$\frac{{\mathit{dq}}_t}{d_t}=K_2{(q_{\text{e}}-q_t)}^2$$ where K₂ (g mg⁻¹ min⁻¹) is the second-order rate constant. Integrating Eq. (20) for the boundary conditions q_t = 0–q_t at t = 0–t is simplified as can be rearranged and linearized to obtain:

(21)

$$\frac{t}{q_t}=\frac{1}{K_2{q}_{\text{e}}^2}+\frac{1}{q_{\text{e}}}(t)$$ The second-order rate constants were used to calculate the initial sorption rate, given by the following equation:

(22)

$$h=K_2{q}_{\text{e}}^2$$ The plot of t/q_t versus time is a straight line as shown in Figs. 6c and 6d. The K₂ and q_e values determined from the slopes and intercepts of the plot are presented in Table 5 along with the corresponding correlation coefficient. This procedure is more likely to predict the behavior over the whole range of adsorption. The linear plot shows a good agreement between the experimental and calculated q_e values (Table 5). The corresponding correlation coefficient (R²) values for the pseudo-second-order kinetic model were greater than pseudo-first-order kinetic model for all CV concentration, indicating the applicability of the pseudo-second-order kinetic model to describe the adsorption (Santhi et al., 2010). It suggested that the adsorption process was controlled by chemisorptions (Hameed, 2009).

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Figure 6c

Pseudo-second-order kinetics at different initial concentrations for CV onto CVM.

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Figure 6d

Pseudo-second-order kinetics at different initial concentrations for CV onto MVM.

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3.6.3

3.6.3 Intraparticle diffusion

When the diffusion (internal surface and pore diffusion) of dye molecule inside the adsorbent is the rate-limiting step, then adsorption data can be presented by the following equation:

(23)

$$q_t=K_{\text{dif}}{t}^{1/2}+C$$ where C (mg g⁻¹) is the intercept and K_dif is the intraparticle diffusion rate constant (in mg g⁻¹ min^−1/2). The values of q_t were found to be linearly correlated with values of t^1/2 (Figs. 7a and 7b) and the rate constant K_dif directly evaluated from the slope of the regression line (Table 6). The values of intercept C (Table 6) provide information about the thickness of the boundary layer and the resistance to the external mass transfer increase as the intercept increase (Santhi et al., 2010). The constant C was found to increase from 5.9603 to 19.246 and 5.8535 to 21.66 with increase in CV concentration from 50 to 200 mg L⁻¹ onto CVM and MVM, which indicating the increase of the thickness of the boundary layer and decrease of the chance of the external mass transfer and hence increase of the chance of internal mass transfer. The R² values are given in Table 6 are close to unity indicating the application of this model. This may confirm that the rate-limiting step is the intraparticle diffusion process. The linearity of the plots demonstrated that intraparticle diffusion played a significant role in the uptake of the CV onto CVM and MVM. However, as still there is no sufficient indication about it, Ho (2003) has shown that if the intraparticle diffusion is the sole rate-limiting step, it is essential for the q_t versus t^1/2 plots to pass through the origin, which is not the case in Figs. 7a and 7b, it may be concluded that surface adsorption and intraparticle diffusion were concurrently operating during the adsorbate and adsorbent interactions.

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Figure 7a

Intraparticle diffusion plot at different initial dye concentration for CV onto CVM.

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Figure 7b

Intraparticle diffusion plot at different initial dye concentration for CV onto MVM.

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Table 6 Parameters of the intraparticle diffusion model for the adsorption of CV onto CVM and MVM.

Adsorbents	C0 (mg L−1)	Kdif	C	R2
CVM	50	0.2189	5.9603	0.9753
	100	0.1830	13.065	0.9854
	150	0.1494	17.272	0.9759
	200	0.1840	19.246	0.9646
MVM	50	0.3373	5.8536	0.9654
	100	0.3651	12.774	0.9618
	150	0.3859	16.559	0.9545
	200	0.2835	21.660	0.9681

3.7 Desorption studies

Regeneration of the adsorbent may make the treatment more economical and feasible. Desorption studies help to elucidate the mechanism of dye adsorption and recycling of the spent adsorbent and the adsorbate. If the adsorbed dyes can be desorbed using neutral pH water, then the attachment of the dye to the adsorbent is by weak bonds. The maximum desorption of CV onto CVM (24.63%) and CV onto MVM (34.27%) is obtained at 2 and 3 pH, respectively (Fig. 8).

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Figure 8

Desorption of CV onto CVM and MVM.

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3.8 Adsorption mechanism

The FTIR studies of the CVM and MVM have confirmed the presence of hydroxyl groups and carboxylic groups. The biosorption of CV onto CVM and MVM may be likely due to electrostatic attraction between these groups and the cationic dye molecules (CV⁺). At pH above 4.4, the carboxyl groups are deprotonated and as such are negatively charged. These negatively charged carboxylate ligands (–COO⁻) can attract the positively charged CV molecules and binding can occur. Thus the CV⁺ binding to the CVM and MVM may be ion-exchange mechanism, which may involve electrostatic interaction between the negatively charged groups in the cell walls and the dye cationic molecules. There is the weak electrostatic interaction between the CV and the electron-rich sites of the adsorbent surface.

Activated carbons are materials which have amphoteric characteristics, so the pH on its surface always changes depending on initial pH of the solution. Generally, the adsorption capacity and rate constant have the tendency to increase as initial pH of the solution increases. This is due to the pH_zpc of the adsorbents which has an acidic value; this is favorable for cation adsorption (Prahas et al., 2008). It is commonly known fact that the anions are favorably adsorbed by the adsorbent at lower pH values due to presence of H⁺ ions. At high pH values, cations are adsorbed due to the negatively charged surface sites of the adsorbents: $${\text{COO}}^{-}+{\text{CV}}^{+}=\text{COO}''–''\text{CV}$$